Hanh mitakuyapi / Hello my relatives. More likely they were grinding the seeds of plants such as lamb's quarters, cattail, and similar at that time. I'm pretty sure lamb's quarters is indigenous to Turtle Island.. The seeds are very high in protein & make a tasty meal once they are ground & soaked.. Also good to thicken stew.. Regardless, there are many plants other than maize that made up our dietary mainstays back then. For that matter, there was the forerunner of maize, a grass, but can't recall which one. Still, the seeds would be edible & grindable.. There's a grass that grows in the marshes out here that looks kind of like a rice, but isn't 'wild rice' as we know it now.. I've gathered it's seeds, parched & ground them & they were pretty decent eating.. Keep up the good work. Nice photos!